The main objective of this study is to 1) determine the dose-response range of Norwalk virus (NV) infectivity in adult human volunteers and 2) begin to define whether ondansetron may be a potential treatment for acute viral gastroenteritis. It is likely that a significant portion of "acute gastrointestinal illness of unknown etiology" episodes are related to NV and that Norwalk and related viruses may be responsible for up to 23% of waterborne outbreaks of acute gastronenteritis. Use of ondansetron, an FDA approved drug for antiemesis in chemotherapy and post-operative procedures, may not only ameliorate symptoms of infection, but may also prevent spread of infection by decreasing vomiting episodes.